The present invention relates to a method of manufacturing a double-walled tube for transporting solid materials, and also to a double-walled tube.
Double-walled tubes are used, for example, in mining underground operations for pneumatic transportation of smallpiece minerals which are brought in the excavated longwall face space as filling material. Moreover, the double-walled tubes are provided during hydraulic transportation of sands for use particularly in the case of excavating works. The properties of the solid material to be transported require tubes with a wear-resistant inner surface having a long service life. On the other hand, the same tubes must be designed so that they grow under the action of inner pressure loads and particularly during transportation of construction materials to the place of use under the action of rough impact loads. For satisfying both these extreme requirements, for years double-walled tubes have been utilized in which a core tube is designed especially for wear loads, whereas a casing tube is designed for pressure and impact loads. It is required that the core tube and the casing tube are fixedly connected with one another. One proposal for manufacturing such double-walled tubes is disclosed in the DE-AS No. 1,099,284. In accordance with this method, the core tube is expanded in the casing tube under the action of cold deformation and simultaneously fixed relative to the outer surface. The DE-PS 858,615 proposes a method in accordance with which a tube is composed of a plurality of steel layers whose heat expansion coefficient decreases from inside outwardly. In accordance with the DE-PS 712,586, a method is proposed in accordance with which in a two-layer tube a glass-hard inner layer is welded with a hardenable tough outer layer. The same source also describes a high-hardened wear-resistant core tube over which a sheet of a tough non-hardenable metal is bent to form a slotted casing tube, and the casing tube is finally welded along its longitudinal edges. The casing tube is shrunk on the core tube because of welding and provides for the desired immovable assembly. Moreover, core tubes of slotted springy wear-resistant steels are used whose slotted edges prior to the insertion into a casing tube of a tough steel are overlapped. The slotted core tube is then expanded by the inner pressure and pressed against the casing tube, whereas finally the slot edges abut against one another. This method is disclosed in the DE-PS 593,599. In the same source, it is disclosed that a pressure expansion is also possible without longitudinal slots. The expansion of the core tube in the casing tube is carried out by a respective tool.
Finally, in accordance with a method proposed in the DE-OS 2,905,071, a slotted core tube of a hardenable tube is used whose outer diameter prior to insertion into a casing tube is greater than the inner diameter of the casing tube. The slot width and the outer diameter are so determined relative to one another that after the pressing-in of the core tube, because of radial compression, the return spring effect provides for firm lying of the core tube in the casing tube with tight abutment of the slot edges. After this, hardening of the core tube takes place, and volume increase resulting from the hardening of the core tube no longer presses against the casing tube. In addition, there is also a possibility to cool the casing tube similarly, to increase the pressing force.
The above discussed developments, which have taken place over several decades, make clear that the experts have not in practice been able to come to a completely satisfactory concept for manufacturing a double-walled tube with a wear-free core tube and a tough casing tube. All proposals as a whole, regardless of whether slotted or unslotted tubes are used, deal with a core tube and a casing tube which are designed such that the outer diameter of the core tube only insignificantly deviates from the inner diameter of the casing tube. Only by maintenance of these narrow tolerances do the experts see a possibility to press the core tube in the required manner against the casing tube. The expenditures both related to accurately manufacture of tubes and also to the devices for insertion of the tubes into one another and hardening are relatively high.